The largest oil reserves in the world are primarily heavy oil. Heavy oil is defined as an oil with an API gravity between 5 and 20. They are typically high in sulfur content, in heavy metals and the bulk of it has a boiling point greater than 524 C. An upgrading facility is required to convert this heavy oil into lighter oil. Generally speaking heavy oils are hydrogen deficient, this is to say, they have more carbon than hydrogen. To upgrade this heavy oil there are two options; one is to remove the excess carbon and this is called coking, the other is by adding hydrogen and this is called hydrogenation. The carbon rejection processes typically produce liquid yields between 75 and 85% were as hydrogenation processes yield liquid volumes greater than 100%.
There are many commercial coking processes as well as hydrogenation processes throughout the world treating feedstocks from raw crudes to crude residuals. The demand for lighter fuels has created a need for improvements in processing the heavy crude.
The main problems present in hydrogenation processes are mass transfer and catalyst poisoning. To increase mass transfer hydrogen must be better mixed with the oil feedstock, it can be done by operating at higher pressures or by generating a turbulent flow. To decrease catalyst poisoning the heavy metals must be removed upstream of the catalyst or the poisoned catalyst removed on a continuous stream.